How the echolocation of bats has shaped their skulls

Bats are some of the most highly specialised mammals to have ever evolved.

This includes not only the evolution of flight, but also their echolocation. This ability requires the bats to produce high-frequency noises, receive the echo back, then interpret it to enable them to build up a detailed picture of the world through sound.

But how this specialised sense has influenced the size and shape of the bat’s skull is not well understood. Now a new study has looked at the association between echolocation and skull shape.

It turns out that the shape of the skull in bat species is related to the specific frequencies of sonar that they produce. This in turn is often influenced by the way in which they emit sound for echolocation.

Dr Carlo Meloro is a palaeobiologist based at Liverpool John Moores University who looks at how mammals evolved.

“This paper is trying to analyse the association between the bat skull morphology and the adaptation to emit a different type of frequency,” explains Carlo, who co-authored this latest study. “We now know that there is an association between different skull shape in bats and the way they feed and how they emit sound.”

“That’s because in bats, there is a clear dichotomy: some species emit the sound using the mouth, and other species instead use the nose.”

The researchers took scans of hundreds of bat skulls found in natural history collections across Europe, including those in the collections that we care for. They found that those species which use higher frequency sonar had relatively shorter faces, while those which emitted lower frequencies had larger ear bones.

The results have been published in the journal Royal Society Open Science.

Mouth emitters vs. nasal emitters

Most bats produce their echolocation sonar using their larynx. But there are two main ways in which bats then emit their sonar.

One way to emit the sonar signals is through the mouth. Known as ‘mouth emitter’, these bats typically have a short face tilted upwards. They generally fly with an upward position of the head facilitating the sound emission.

The second are those called ‘nasal emitters’, which project their sonar through their noses. These bats usually have relatively elongated face, bigger ear bones and often elaborate nasal discs like those seen in horseshoe and leaf-nosed bats.

Carlo and his colleagues thought that even if these two groups can use similar frequencies, they would differ in skull and facial morphology associated with shaping and directing the sonar beam.

“We also validated the impact of frequency emission on skull sizes,” explains Carlo.

“This relationship had already been anticipated by several researchers, who proposed that the skull can function like an acoustic resonator. Because high‑frequency sounds have very short wavelengths, they are more efficiently produced and received by smaller cranial and facial structures, whereas lower‑frequency sounds interact better with larger anatomical cavities.”

“In this way, skull size can evolve to ‘match’ the typical frequencies a species uses.”

Consistent with this expectation, the team found that species using higher frequencies typically possessed smaller skulls, and those using lower frequencies tended to have larger ones. In addition, they discovered that bats emitting higher frequencies had disproportionately wider ear bones, reflecting the need for heightened sensitivity to the very short‑wavelength echoes such calls produce.

The researchers also suspected that the diet of the bats in question might have an influence on how echolocation changed the skull shape. This is because while insectivorous bat species need to have a highly tuned echolocation to precisely locate fast-moving insect prey flying around in the dark, this should be less important for fruit-eating species.

Rather than solely relying on echolocation to find fruit, these bats are also using a range of other senses such as sight and smell. As a result, it was expected that the selection pressure on their skulls from echolocation would not be as extreme as that seen in insect-eating species.

“Traditionally, people have always suggested that frugivore bats generally use multiple sensory systems,” says Carlo. “But what we found was a bit unexpected.”

“We identified a consistent association between the skull shape and the frequency peak in fruit-eating bats. Remarkably, frugivores showed a pattern similar to that of insectivorous nasal emitters in that species using higher frequencies tended to have relatively shorter faces. This suggests that the biomechanical and acoustic constraints imposed by high‑frequency sound production act in a comparable manner across very different ecological groups.”

Carlo suspects that this might be because even though the fruit-eating bats rely less on echolocation to find their food, they still need to navigate cluttered vegetation and land precisely on fruiting branches. This shows that selection for acoustic performance appears to shape skull morphology even in species where echolocation is not the primary sensory modality.